I recently needed to supply two variations of an algorithm in an F# source file – one for .NET 3.5 consumers and another for .NET 4.0 consumers (the later framework version allows parallel execution using the FSharp.Collections.ParallelSeq). One way is to tell msbuild to reference the library, conditional on the version of the .NET framework – and also, define a constant which can be used within the source code to choose the algorithm.